1. Field of the Invention
The present invention is generally in the field of temperature stabilization for semiconductor devices. In particular the present invention is in the field of temperature stabilization in flip chip technology.
2. Background Art
As the performance, speed, and complexity of semiconductor devices increase, semiconductor devices tend to increasingly generate significant amounts of heat. Moreover, the continually shrinking packaging containing semiconductor devices has contributed to a reduction of the ability of these devices to dissipate heat through convection. The space surrounding these devices has become significantly more confined as packaging sizes shrink, thereby reducing the opportunity for convection currents to circulate and remove heat.
In addition, the increasing popularity of portable electronic devices such as cellular phones, portable televisions, palm top computers, and pagers has contributed to a demand for using smaller packages made of lighter material such as plastics which are generally lighter than metals. Plastics, relative to metals, however, generally tend to have a greater resistance to heat transfer. The opportunity for heat transfer, and the cooling of the power circuitry via conduction, is thus significantly reduced by the increasing use of nonmetallic and plastic packaging materials.
Reliability of semiconductor devices is related to the temperature of the devices. Manufacturers of portable electronic systems have sought to reduce the amount of heat generated by the semiconductor devices within those systems, and to spread the heat that is generated, in order to reduce peak temperatures which would affect the reliability of the semiconductor devices within those systems. Manufacturers have made efforts to reduce or spread the heat specifically within power devices, which tend to generate a significant amount of heat.
Some manufacturers of power devices have taken the approach of adding metal heat sinks to their power devices. However, the effectiveness of the heat sinks diminishes with the air volume available for convection cooling surrounding the heat sink. Thus, the increasingly small size of portable electronic devices, as well as the size reduction of the semiconductor packaging itself, have reduced the effectiveness of heat sinks.
Another method of reducing power consumption, and therefore heat generation, is to employ a digital design. Digital communication systems are, in large part, replacing analog communication systems. This is so because digital systems, generally, can offer increased performance and lower overall power consumption than analog systems. Digital systems commonly operate in a time sharing mode or pulse mode. That is a digital system will turn on, broadcast data and then turn off. This time sharing mode allows several communications systems to share the same frequency without interfering with each other. A time sharing system can also lower overall power dissipation of a communication system, because it operates for only a fraction of the time that a continuous system operates.
The rapid cycling on and off of the output of the time sharing system can, however, give rise to significant peak power dissipation. The rapid power cycling of devices can lead to continual thermal stress as the devices are turned on, dissipate considerable power, and then are turned off. In the confined space of a personal communication device, such as a portable telephone, the temperature swings due to the rapid cycling of power can lead to significant, continuous thermal and mechanical stress on the semiconductor devices, circuit connections, wire bonds, and other mechanical connections. As stated above, portable electronic devices cannot house heat sinks to reduce the temperature swings due to the rapid cycling of power.
Thus, there is a need to reduce the thermal and mechanical stress, induced by power cycling, to increase overall reliability in digital time sharing or pulse mode communication systems and other power cycling digital systems. In other words, there is serious need in the art to reduce the magnitude of temperature excursions due to the rapid cycling of power in digital systems.
The present invention is method and structure for temperature stabilization in flip chip technology. The invention results in a reduction in the range of temperature excursions in a semiconductor die mounted on an interconnect substrate utilizing the flip chip technology. The reduction in the range of temperature excursions results in a reduction of thermal and mechanical stress during operation of the semiconductor die and thus improves the performance and reliability of the semiconductor die. In particular, the invention improves the overall reliability in digital time sharing or pulse mode communication systems and other power cycling digital systems.
The invention utilizes and incorporates a Phase Change Material (xe2x80x9cPCMxe2x80x9d) to reduce the range of temperature excursions in a semiconductor die attached to an interconnect substrate in the flip chip technology. In one embodiment of the invention a PCM underfill, which comprises PCM microspheres interspersed within a polymer, is dispensed in the interface area between the semiconductor die and the interconnect substrate.
Reduction of the range of temperature excursions in the semiconductor die is achieved since the PCM underfill acts as a cushion to dampen the range of temperature excursions of the semiconductor die. During dissipation of power pulses in the semiconductor die, the PCM underfill absorbs energy from the semiconductor die by changing phase from solid to liquid without a concomitant rise in the temperature of the PCM underfill. Thus, the energy released when power pulses are being dissipated in the semiconductor die does not result in a rise in the temperature of the PCM underfill. Accordingly, the temperature of the semiconductor die which is in thermal contact with the PCM underfill is not abruptly increased during power pulses.
Similarly, during the time that no power pulse is being dissipated by the semiconductor die, the PCM underfill releases the stored energy by changing phase from liquid to solid while maintaining a constant temperature. Thus, the temperature of the semiconductor die which is in thermal contact with the PCM underfill is not abruptly decreased when no power pulse is being dissipated in the semiconductor die. In this manner the range of temperature excursions in the semiconductor die is dramatically reduced by the present invention.